Thin film processes developed in the field of microelectronic integrated circuits have been used to produce precision microelectromechanical devices. For example, solid state laser and fiber optic couplings, ink jet nozzles and charge plates, magnetic disk read/write heads, and optical recording heads have been produced using thin film processes including photolithography, sputter deposition, etching, and plasma processing. These thin film processes allow the production of microelectromechanical devices with submicron dimensional control.
One important microelectromechanical device is an electrostatically driven rotating mirror which is used in an optical scanner such as a bar code reader. In particular, an electrostatically driven torsional scanning mirror is discussed in the reference entitled "Silicon Torsional Scanning Mirror" by Kurt E. Petersen, IBM J.Res.Develop., Vol. 24, No. 5, September 1980. In this reference, a single-crystal silicon chip contains a mirror element attached to two single-crystal silicon torsion bars. This silicon chip is bonded to another substrate into which a shallow rectangular well has been etched. At the bottom of the well, two electrodes are alternately energized to deflect the mirror element in a torsional movement about the silicon torsion bars.
The silicon torsion bars, however, may be unnecessarily stiff thus requiring excessive torque to rotate the mirror. In addition, the location of the electrodes in the path of the rotating mirror may restrict the rotation of the mirror. Increasing the distance between the electrodes and the mirror may reduce the electrostatic force generated therebetween. Furthermore, the bonding of the silicon chip to the second substrate may add unnecessary complication to the fabrication of the device.
A two-dimensional optical scanner is discussed in the reference entitled "2-Dimensional Optical Scanner Applying a Torsional Resonator With 2 Degrees of Freedom" by Yoshinori Ohtuka et al., Proceedings, IEEE Micro Electra Mechanical Systems, 1995, pp. 418, 306-309. This reference discusses a torsional vibration system where two vibration forces are produced by one driving circuit. In particular, bimorph cells are used to excite the torsional vibration. One-dimensional scanning is enabled by driving the bimorph cells with the resonance frequency of either of the two torsional vibrations. Two-dimensional scanning can be achieved if the bimorph cells are operated by adding the resonance frequency signals of the two torsional vibrations. The scanner of this reference, however, may only be able to independently scan in any one dimension at predetermined resonance frequencies. In other words, because a single driving circuit is used to excite vibration about two axes, vibration about either axis may be limited to predetermined resonance frequencies. The scanner of this reference may also require the assembly of discrete components.
Notwithstanding the above mentioned references, there continues to exist a need in the art for improved microelectromechanical scanners and methods.